Many electronic devices, such as computers and many household appliances, require one or more regulated DC voltages. The power for such electronic devices is ordinarily supplied by power converters that convert an AC line voltage into the regulated DC voltages required by the devices. A significant portion of the world uses an AC voltage standard of either 120 Vrms or 220 Vrms. A few areas set their voltage standard at 240 Vrms.
Electrical power converters commonly include a rectifier circuit which converts the AC line voltage to an unregulated DC voltage, also known as a rectified line voltage, and a DC-to-DC converter for convening this unregulated DC voltage into one or more regulated DC output voltages. The maximum nominal unregulated DC voltage generated by such a rectifier circuit will typically be 340 V to 385 V for a 240 Vrms AC voltage input.
Other power supplies use power factor correction circuitry instead of a simple rectifier circuit to raise the power factor of the unregulated DC voltage and eliminate harmonic distortion. Such circuits are often referred to as power factor correction ("PFC") circuits.
A difficulty with rectifier or PFC circuits is that they allow input transient voltages lasting tens of milliseconds and having an amplitude of 450 V or more to be produced. For example, a transient AC voltage of 317 Vrms lasting 20 ms may appear on the input AC line. The resultant rectified DC voltage that will be supplied to the DC-to-DC converter will be dangerously high, approximately 450 V DC. This voltage will typically be significantly above the maximum input voltage rating for the converter. In known prior art systems, in order to prevent such transients from damaging the converter, unsophisticated transient energy absorption devices are used to blow a fuse. The problem with such solutions is that, when the fuse blows, it renders the unit inoperable.
An alternative method for responding to the occurrence of a dangerous transient overvoltage condition in a system using a PFC circuit would be to disable the PFC circuit until the overvoltage condition has subsided. Typical PFC circuits have a disable pin that is accessible to a control circuit. Unfortunately, if such a disable pin were used, the PFC module is forced to go through a start up scenario that could take seconds, during which time the power system would have collapsed.
Accordingly, what is needed is some technique to allow operation of the converter to continue, not only after the high level DC transient voltage pulse has subsided, but also during the time the DC transient voltage is being produced by the rectifier circuit, without damage to the converter or any system components.